It is desirable in various applications to confer hydrophobicity and/or lipophobicity to a surface of a solid substrate. Fluorosilane oligomers, such as water-born Evonik's Dynasylan® F8815, are effective for altering, temporarily or permanently, the hydrophobicity and/or lipophobicity of the surface of a wide range of substrates such as glass, wood, concrete and fabrics. However, the low solubility in water of most commercial fluorosilane oligomers means that they are mostly sold and used dissolved in a non-aqueous organic solvent. This brings significant restrictions on these products' applications because of transfer regulations, environmental hazards and health concerns.
On another subject, self-propulsion of a liquid occurs when a surface tension gradient causes the liquid to flow, for example against the gravitational force. Indeed, a difference in surface tension on either side of a liquid droplet produces a directional transport of the droplet through a second liquid or over a solid substrate. This phenomenon is known as the chemical Marangoni effect.
A classic example is wine, which may exhibit a visible effect called “tears” on the wall of a glass, as shown in FIG. 1A). This effect is due to the fact that alcohol has a lower surface tension and higher volatility than water. The water/alcohol solution (i.e. wine) first rises on the surface of the glass due to capillary action. Then, the alcohol evaporates from the film leaving behind a liquid with a higher surface tension (because it contains more water and less alcohol). This region with a lower concentration of alcohol (greater surface tension) pulls on the surrounding fluid more strongly than the regions with a higher alcohol concentration (lower in the glass), see FIG. 1B. As a result, the liquid is pulled up until its own weight exceeds the force of the Marangoni effect, and the liquid drips back down the vessel's walls. The Marangoni effect can also be easily demonstrated by spreading a thin film of water on a smooth surface and then allowing a drop of alcohol to fall on the center of the film. The liquid will rush out of the region where the drop of alcohol fell.
In artificial systems, a similar effect can be achieved for a droplet sitting on a homogeneous surface if the droplet contains a species which adsorbs onto the surface. Droplet motion is then driven by the irreversible modification of the surface free energy which affects the interfacial energy on either side of the droplet—a so-called “chemical” Marangoni effect. The energy released by chemical reactions with the surface that cause the droplet of liquid to move across the surface and even up a non-horizontal surface.
The self-propulsion of liquid droplets on solid surfaces has attracted broadening attention for decades. This droplet motion system can be an exceptional model of the process of chemo-mechanical energy conversion. However, all the reported “chemical Marangoni” effects observed with fluoro-organosilanes are for driving oil droplets. Limited by the critical micelle concentration and organo-silanes solubility in water, it is difficult to produce self-propelling water droplets. Thus, it remains challenging to develop an aqueous self-propelling system.